1. Field of the Invention
The present invention relates generally to fluid shear couplings in which the coupling of a drive member and a driven member is variable, and more particularly to a fluid shear coupling responsive to preselected parameters.
2. Description of the Prior Art
Fluid shear couplings have been used in the prior art for a great variety of mechanical applications. The shear properties of a contained, viscous fluid are utilized to provide a varying degree of coupling between a drive member and a driven member, which members are typically mounted for coaxial rotation. A primary field of application for such fluid shear couplings is in the area of automobile engine cooling systems. Internal combustion engines generally utilized in automobiles are required to be cooled to maintain the engine below a certain maximum temperature. The engine cooling is typically obtained by pumping a fluid through passageways in the engine block with the fluid being transported to a radiator which provides heat dissipation by radiation and convection.
The cooling requirements for an automobile engine vary based upon several factors, including the speed of the automobile, the ambient temperature, and the speed at which the engine is operating. Fluid shear couplings used in connection with automobile engines are typically designed to have a drive member coupled to the engine and a driven member adapted to carry a fan for moving air over the radiator. The fluid shear coupling may be utilized to provide an appropriate rotation of the fan with respect to the conditions relevant to the required engine cooling.
Several factors are relevant to the construction and operation of fluid shear couplings. The degree of coupling between the drive member and the driven member depends upon many factors, including the proximity of the surfaces of the respective members which define the working chamber and the amount of working fluid contained within the working chamber. In the usual machining techniques for forming the coupling elements, the radial clearances between the coupling disc and housing may be achieved more closely than the axial clearances. Greater precision for radial clearances and more controlled coupling action is therefore generally more obtainable if the coupling uses primarily radial clearances to obtain the coupling action. The rate of movement of working fluid into and out of the working chamber, or in other words the rate of engagement and disengagement of the coupling action, is relevant to the accuracy with which a preselected temperature may be maintained. In effecting coupling of the coupling disc and housing, there is a substantial generation of heat within the working fluid and surrounding structures due to the considerable slippage of the two members and consequent friction. The rate of coupling and disengagement is also related to the generation of heat during these transitional phases of the fluid shear coupling. With this heat being generated, it is highly desirable to provide for dissipation of the heat to prevent working fluid and material degradation and fatigue. This heat dissipation is generally accomplished by convection from cooling fins located on the housing, and it is therefore advantageous to provide many such fins as close to the working surfaces as practical. Another consideration for fluid shear couplings is the size of the unit, especially the diameters of the rotatable coupling disc and housing, since these couplings frequently must fit in a confined area, and compactness also contributes to lower material costs. It is also desirable to avoid the build-up of deposits from the working fluid, which may be accomplished by circulating the working fluid during coupling of the housing and coupling disc.
In U.S. Pat. No. 3,559,785, issued to Weir on Feb. 2, 1971, there is disclosed a variable fluid coupling particularly adapted for use in connection with an internal combustion engine. The Weir coupling includes a coupling disc rotatably received within a housing. A first side of the coupling disc includes an annular cavity which operates as a storage chamber for the working fluid utilized in the coupling. This storage chamber communicates through an aperture to the second side of the coupling disc and a bimetallic strip is located on the second side to close the aperture in response to the ambient temperature. The coupling disc of the Weir device includes grooves on the sides and periphery of the coupling disc to cause a continuous flow of the working fluid around the coupling disc to the storage chamber. The Weir coupling is constructed to provide a more limited flow of the working fluid when the ambient temperature is reduced and the bimetallic strip consequently closes the aperture communicating with the storage chamber. This reduced flow of working fluid provides for a reduced coupling between the coupling disc and the housing and thus reduces the rotation of the fan during periods of cooler ambient temperatures. The Weir coupling is therefore temperature responsive and provides for varying rates of fan rotation depending upon the ambient temperature. The Weir coupling, however, does not provide for directly correlating the rate of fan rotation with the temperature within the engine block, which temperature is believed to be more relevant to the desired rate of rotation of the fan. Further, the coupling disc of the Weir coupling does not provide for a maximum efficiency in dissipating the heat generated within the working fluid during coupling of the drive member and the housing.
A temperature and speed sensitive drive coupling is disclosed in U.S. Pat. No. 3,059,745, issued to Tauschek on Oct. 23, 1962. The Tauschek coupling includes a rotatable clutch plate mounted coaxially within a housing. The flat periphery of the clutch plate is received within a narrow working chamber defined by the housing, and working fluid received within the working chamber operates to couple the clutch plate with the housing. The housing further defines an annular chamber disposed at the periphery of the clutch plate for reception of the working fluid. An expansible element is received within the annular chamber and is operable to expand and move fluid into the working chamber upon an increase in temperature. At the same time, the expansible element is responsive to the centrifugal force generated within the working fluid as the rotational rate of the housing increases. The expansible element may be selected in the Tauschek coupling to cause a reduction in the amount of working fluid received between the clutch plate and the housing, thereby reducing the degree of coupling between the two, when the rotational rate of the housing reaches a preselected upper limit. The Tauschek coupling is therefore both temperature and speed responsive. However, the Tauschek coupling does not directly correlate the rotation of the driven member to the temperature of the engine to which it is mounted, and further, because of the configuration of the clutch plate, does not provide for maximum efficiency in the dissipation of heat generated within the fluid. Fluid shear couplings having substantially the same construction are disclosed in U.S. Pat. Nos. 3,727,735 issued to La Flame on Apr. 17, 1973, and 3,088,566, issued to Flemming on May 7, 1963.
A temperature and/or speed sensitive fluid shear coupling is disclosed in U.S. Pat. No. 3,983,980, issued to Weintz on Oct. 5, 1976. The Weintz coupling includes a coupling disc having a wall extending from its periphery. The wall is sloped inwardly toward the center of the disc to provide, together with the disc, a working fluid reservoir. An aperture extends from this reservoir to the opposite side of the disc to provide fluid communication therebetween. A speed or temperature sensitive slot member is positioned to close the aperture in the disc and to thereby limit the flow of fluid from the reservoir through the aperture. The outer surface of the inwardly-sloped wall defines a groove for causing the working fluid to circulate around the wall and into the fluid reservoir. Although the Weintz coupling does provide for temperature-sensitive response, it does not correlate this response to the temperature within the engine. Moreover, the dissipation of heat from the drive fluid is impaired because the inwardly-sloped wall precludes the location of cooling fins in the full proximity of the working fluid.
Another prior art fluid shear coupling is disclosed in U.S. Pat. No. 3,170,552, issued to Mitchell on Feb. 23, 1965. The Mitchell coupling includes a coupling disc having an enlarged periphery which is somewhat conical in cross section. The surfaces of the enlarged periphery of the coupling disc define a working chamber with the housing within which the coupling disc is received. A fluid reservoir is positioned radially outward of the enlarged periphery of the coupling disc and is operable to move fluid into the working chamber. The fluid reservoir includes a diaphram which is spring-biased toward the center of the coupling disc, but which yields under a sufficient centrifugal force within the fluid to permit the fluid to move outwardly into the reservoir. Again, the Mitchell coupling is not responsive to a temperature within the engine. The Mitchell coupling does not provide a somewhat greater amount of working surfaces in the coupling disc for the given radial extent of the disc than had been provided in certain other prior art structures. However, the use of an enlarged periphery of the coupling disc again limits the efficiency of dissipating the heat from the working fluid.
In U.S. Pat. No. 3,323,623, issued to Roper on June 6, 1967, there is shown another construction for a fluid shear coupling. The Roper coupling includes a coupling disc which forms a working chamber with the housing within which it is received. The coupling disc of the Roper coupling includes several cylindrical flanges which extend from one side of the disc, giving the disc a comb-like appearance in cross section. The Roper coupling does provide the advantage of obtaining a relatively large amount of working surface for a given radial extent of the coupling disc. However, the dissipation of heat from the working fluid is limited due to the close proximity of the several flanges on the one side of the coupling disc.
As previously described, several factors are important in the construction and operation of fluid shear couplings. Circulation of the working fluid is advantageous to inhibit build-ups of deposits which would interfere with operation of the coupling, and to distribute generated heat to avoid fluid and material degradation. It is also desirable to provide an increased amount of working surface for a given radial extent of the coupling disc, while not sacrificing a quick rate of coupling and disengagement and an efficient rate of heat dissipation. While the various fluid shear couplings of the prior art are well suited to particular applications, a fluid shear coupling combining all of the above advantages has not previously existed.